Prior art power transmitting apparatus usually include an electromagnetic clutch having for example an electromagnet and pilot clutch, a boosting mechanism formed by a cam, and a main clutch formed by a wet multiple disc clutch. A fastening torque is caused in the pilot clutch by energizing the electromagnet. A fastening force of the main clutch is caused by the boosting mechanism in accordance with the capacity of the fastening torque. The main clutch is connected by its fastening force (i.e. clutch plates and clutch discs of the multiple disc clutch are connected) to transmit the rotational power of the input member to the output member.
The reaction force of the boosting mechanism, resisting force against the fastening force applied to the main clutch, is supported by a case forming a box of the power transmitting apparatus. The torque capacity of the pilot clutch is controlled by an electric current supplied to the electromagnet. That is, the transmitting torque capacity of the power transmitting apparatus can be controlled by controlling the current supplied to the electromagnet. Such a power transmitting apparatus of the prior art is disclosed for example in Japanese Patent No. 2820161.
However, in the prior art power transmitting apparatus, although it is possible to increase the torque capacity by increasing the current supplied to the electromagnet, the torque capacity is limited since the transmittable torque capacity is saturated when the magnetic flux density in the magnetic path of the electromagnet approaches the saturation magnetic flux density of material of the pilot clutch. In addition, since the electromagnet, pilot clutch, and the boosting mechanism etc. are arranged within the case, the power transmitting apparatus of the prior art is oversized even if compared with a hydraulic multiple disc clutch having the same torque capacity.